IMPROVED UNDERSTANDING OF TWO PHASE FLOW PHENOMENA BASED ON UNSTEADY BLADE PRESSURE MEASUREMENTS

Transcription

1 Journal of Computational and Applied Mechanics, Vol. 2., No. 1., (2001) pp IMPROVED UNDERSTANDING OF TWO PHASE FLOW PHENOMENA BASED ON UNSTEADY BLADE PRESSURE MEASUREMENTS Günter Kosyna, Priyatna Suryawijaya, Jens Friedrichs Pfleiderer Institut für Stömungsmaschinen, TU Braunschweig Langer Kamp 6, Braunschweig, Germany [Received: February 1, 2001] Abstract. Impeller blade and side wall pressure measurements deliver additional information of the energy transfer in a centrifugal pump under two phase flow conditions. This information is used to improve the phenomenological understanding as well astocalibrate numerical two phase flow CFD codes. This paper gives an introduction into the measuring technique based on subminiature pressure transmitters with telemetric data transmission and gives a choice of results, obtained by using this method. Keywords: Two phase flow, unsteady pressure measurement, cavitation, blade pressure distribution. 1. Introduction Two phase flow phenomena in centrifugal pumps are an important research subject even today. The present paper is concerned with two phase flows of two different types: with phase transition: cavitation, without phase transition, i.e. water plus undissolved air. The reason to combine both fairly different types in one paper is the fact that basically the same test technique and the same test pump has been used for the investigation of two phase flow with and without condensation. Throughout this paper the following notations and notational conventions areap- plied: Nomenclature Subscripts d diameter bld blade f frequency d pressure side H pump head opt best efficiency point Mbld local blade pressure momentum s suction side (p bld p s ) A r n rotational speed v vapor n s specific speed 0 in front of inlet NPSH net positive suction head 1 impeller inlet p pressure 2 impeller outlet

2 46 G.Kosyna,P.Suryawijaya,J.Friedrichs Nomenclature Q flow rate r radius t time z number of blades β blade angle η efficiency Cavitation causes noise, deterioration of efficiency and what is most important, damage by erosion to the impeller and other components of the pump. CFD codes normally do not have problems to predict the inception of cavitation, but it ismuch more difficult to predict the NPSH for a head drop of 3%, which is the most common cavitation criterion for pumps in industrial applications. At a head drop of 3%, cavities are extended, unsteady cloud cavitation may occur. Pump engineers know very well that the change in pump head is not a reliable indicator for the actual type of cavitation and for the pump wear. 1.4 H H opt P P opt 1.2 η η opt 0% 1% 3% 3% 7% 5% % 7% 5% 5% 1% Q/Q opt 3% 0% 1% Figure 1. Influence of the gas content on the performance of the centrifugal pump 0% Cavities for example may produce an extra lift, that means an increase of the pump head as long as the cavities are attached [1], other phenomena like fluctuations or rotating cavitation may cause an extra head drop. For a better understanding it is necessary to get a correlation between the type of cavitation on the one hand and on the change of pump head and on erosive attack on the other hand. Though the flow with extended and unsteady cavities is very complex, the development of numerical codes showed large progress in the last few years - but of course there is a strong demand for detailed experimental results for validation. In a wide range of pump applications, i.e. in the chemical industry or in the offshore oil production the liquid to be pumped contains certain contents of undissolved gas. It is well known that the performance of standard centrifugal pumps decreases rapidly in liquid/gas two phase flow - the deterioration starts from a gas void fraction of 2-3% - the total breakdown of pumping can be expected at gas contents of 8-15% (Figure 1). For two phase flow applications of centrifugal pumps the influence of the gas void fraction on the pump performance must be predictable - best of all with a suitable nu-

3 Two phase flow phenomena based on unsteady blade pressure measurement 47 merical code. The main problem is a proper modelling ofthe complicated impeller flow structures at higher void fractions with typical separation and demixing area. All attempts to develop a numerical two phase code must be backed up by detailed experiments. 2. Two phase flow test pumps This situation is the background for several present experimental and numerical research projects on two phase flow at German Universities with and without phase transition. The experimental methods used in the projects are: optical observations (CCD, High Speed Photography, laser optical methods) measurements of the erosive attack pressure measurements in the rotating impeller In these projects three test pumps of same geometry but different scale are operated in Braunschweig (see Figure 2.), Darmstadt and Magdeburg - main components of the Magdeburg pump have been built in Miskolc. 12 discharge pipes (2.5") p d vaneless diffuser plexiglas window p s telemetric transmitter pump shaft telemetric receiver impeller Figure 2. Two phase flow test pump The background for the choice of this particular pump design was to have a simple geometry for easy grid generation, for precise machining and good access for optical methods. The first impeller has a one circular arc design of the bladings, the second impeller blading consists of two circular arcs (see Figure 3) in order to obtain a more steady deceleration of the impeller relative flow. Approximately the impeller flow

4 48 G.Kosyna,P.Suryawijaya,J.Friedrichs blade pressure taps (suction/pressure side) Nr. 7 Nr. 8 Imp. 2 Nr. 6 Nr. 5 Nr. 4 Nr. 3 1st circ. arc Nr. 2 Nr. 1 2nd circ. arc hub pressure taps Impeller No. 1 (17205) No. 2 (19235) Blade shape 1-cir.-arc 2-cir.-arc Inlet Diameter d mm 260 mm Outlet Diameter d mm 556 mm Blade inlet angle β 1 17 o 19 o Blade outlet angle β 2 20 o 23 o Passage width b 46 mm 46 mm Number of blades z 5 5 Specific speed n s Rotational speed n 6-12 Hz 6-12 Hz Figure 3. Design parameters of test impellers may be regarded as a 2D-flow. The test pump either can be connected to a closed variable pressure circuit for cavitation tests or to an open circuit with components to mix and separate air for the liquid-gas-two phase flow tests. 3. Blade pressure measurements in the rotating impeller The special task of the Pfleiderer-Institute of the Technical University of Braunschweig in the above mentioned projects is the measurement of the impeller blading acrylic glass shroud alternative measuring tap (sealed) pressure transducer flat seal measuring blade adjustible screw fixing plate tube channel hub silicon oil sealing ring nut reinforcing tube measuring tap transducer cables R Ermeto - tube Figure 4. Installation of subminiature pressure transducer

5 Two phase flow phenomena based on unsteady blade pressure measurement 49 and side wall pressure, using subminiature pressure transducers rotating withthe impeller. The objectives are: a. Cavitation. Investigate the type of cavitation, identify unsteady states of cavitation like fluctuations or rotating cavitation and find correlations between change of pump head and type of cavitation. b. Liquid-gas flow. Improve the understanding of performance drop, deliver calibration data for the development of numerical codes. sub miniature pressure transducers two phase flow telemetric transmitter secondary coil/ transmitter antenne PC ω rotational speed meter triger OT time primary coil/ receiver antenne transient recorder impeller VHF power suply for primar coil telemetric receiver Figure 5. Telemetric pressure data transmission pressure side blade p0 p T - p 0 pd - p 0 pt - p 0 Q/Q ref : 1,0 H : H100% p 0 : bar n : 9 Hz impeller : suction side ps - p 0 measurement calculation Figure 6. Impeller blade pressure and extension of the cavity

6 p blade -p in [bar] s 50 G.Kosyna,P.Suryawijaya,J.Friedrichs p [bar] NPSH = 1.75m Blade suction side p [bar] NPSH = 2.09m Blade suction side Transducer 1 Transducer f [Hz] f [Hz] Ψ Q = 0,45 * Q opt Imp. No 2 (19235) NPSH Figure 7. Frequency analysis of blade pressure signals under different cavitation modes 1.0 Q/Q design = 1.0; 0% gas fraction Q/Q design = 1.0; 3% gas fraction Q/Q design = 1.0; 5% gas fraction pressure distribution pressure side pressure distribution pressure side pressure distribution pressure side M*blade suction side momentum distribution (r-r 1 )/(r 2 -r 1 ) suction side momentum distribution (r-r 1 )/(r 2 -r 1 ) momentum distribution suction side (r-r 1 )/(r 2 -r 1 ) Figure 8. Influence of the gas fraction on blade pressure distribution As the pressure measurement is scheduled for areas with high mechanical impact by cavitation, a robust design has to be developed in order to shelter the sensitive transducers Figure 4. The transducers are installed inside the blade contour, the membrane protected by silicon oil against direct attack of cavitation. This design needs a sufficient blade thickness - this is the reason why the pump used in Braunschweig has double scale compared with the Darmstadt and Magdeburg pumps. Eight pressure transducers are installed inside an impeller blade, four transducers in the impeller hub, the locations of the transducers are shown in Figure 3.

7 blade pressure in [bar] Two phase flow phenomena based on unsteady blade pressure measurement 51 0% gas fraction 3% gas fraction 1% gas fraction 5% gas fraction impeller: 2KB Q/Q = 1.0 opt 7% gas fraction time in [msec] Figure 9. Pressure fluctuations at transducer Nr. 8 at different gas void fractions The signals of 8 transducers can be transmitted simultaneously by using a telemetric system (Figure 5.), type dt 204/301/612 from Datatel Telemetrie Elektronik. This system allows to transmit frequencies up to 2,5 khz, which is sufficient, as it is intended only to detect macroscopic flow phenomena but not high frequency cavitational noise. 4. Results a. Cavitation. In the following a choice of results is presented to demonstrate the potential of unsteady pressure measurement inside the rotating impeller. The first investigation using this technique was completed by Dreiß in 1997 [1] [2]. Two subsequent projects are under way now by the authors of this paper. Figure 6 shows the result of time averaged impeller blade measurements under cavitation assembled with a stroboscopic CCD-snapshot of the cavity. The test results are compared with a numerical calculation by V. Schütte [3]. Figure 7 shows a result from the subsequent project on cavitation. It shows the FFT of the pressure signals transducer, in impeller 2 under 45% part load. The frequency analysis shows the impeller rotating frequency as dominating in the right diagram for NPSH=2.09m (s=1.05). At a slightly lower NPSH (1.75m; s=0.94) value other strong peaks at different frequencies appear - the head drop curve starts its creeping descend. This result indicates that the pressure drop is related to the onset of periodic or rotating cavitation phenomena - this first finding of course needs further investigation for confirmation.

8 52 G.Kosyna,P.Suryawijaya,J.Friedrichs b. Liquid-gas flow. The drop of the pump head with increasing gas void fraction is a result of the change of the impeller blade pressure. Figure 8 shows the drop of the blade pressure rise with increasing gas void fraction, i.e. at 5% gas no pressure rise can be observed at the first 40% of the impeller blade length, which is a result of a demixing of the two phase flow. Consequently there is no blade momentum transferred in the first 40% of blade length either. The character of impeller flow changes with increasing gas void fraction (Figure 9). The pressure fluctuations increase with rising gas percentage, having a maximum at five percent, then decreasing with further increased gas void fraction. 5. Conclusions This paper gives a short introduction to the unsteady impeller pressure measurement technique and presents a choice of typical results obtained with this method. Impeller measurements improve the general understanding of two phase pump flow phenomena and deliver detailed experimental data for further development ofnumerical two phase codes. Investigations will soon be extended to other impeller geometries with higher specific speeds. REFERENCES 1. Dreiß, A.: Untersuchung der Laufradkavitation einer radialen Kreiselpumpe durch instationäre Druckmessungen im rotierenden System, Dissertation TU Braunschweig Dreiß, A. and Kosyna, G.: Experimental investigations of the cavitation states in a radial pump impeller, Proceedings of the JSME Centennial Grand Congress, International Conference on Fluid Engineering, Tokio, Japan, 1997, Vol. 1., Schütte, V.: Bubble and leading edge cavities in centrifugal pumps, 2nd European Fluid Mechanics Conference, Warsaw, Poland, 1994.